Found 44 talks archived in The Galaxy

Abstract

The SDSS Apache Point Observatory Galactic Evolution Experiment (APOGEE) hascollected high resolution near-IR spectra of several hundred thousand starsacross the Milky Way. I'll describe some observational results about thespatial variation of chemical abundances as a function of Galactocentricradius and distance from the midplane, discussing mean abundances, metallicitydistribution function, and the variation of abundance ratios of multipleelements. Additional information related to stellar ages can be obtainedfrom [C/N] for red giant stars. Several lines of evidence suggest that radialmigration has had a significant impact on the Galactic disk. The observed patterns ofabundance ratios may provide observational constraints on nucleosynthetic yields.

Abstract

The Milky Way (MW) galaxy is not much different from its faraway cousins. However, our position within the MW allows us to study the properties of its stellar populations with exquisite detail in comparison to extragalactic sources. The bulge of the MW (i.e. the stellar population within ~3 kpc from the Galactic center) is the most massive stellar component of the MW hosting very old stars (>10 Gyr), therefore the study of its stellar population properties can shed light on the formation and evolution of the MW as a whole, and of other spiral galaxies at large.

So far, there is a general consensus on the global kinematic, chemical and structural properties of the bulge populations, however the age, or rather, the distribution of the ages of the stars in the bulge is yet to be completely understood. We aimed at addressing the questions 'How old is the bulge?' and 'Is there a spatial age gradient in the bulge?' through the determination of the stellar ages in the different fields sparsely distributed within a region of 300 deg² centered on the bulge. We use images from the VISTA Variables in the Vía Láctea (VVV) survey, based in near infrared passbands, to extract accurate magnitude and color of half a billion stars in the bulge area using point spread function fitting. The newly derived photometric catalogs, used in addition to probe the extinction towards the bulge, will be made publicly available to the entire community. The contribution of the intervening disk population along the bulge lines of sight has been detected and removed by using a statistical approach in order to obtain a final stars sample that is representative of the bulge population only. The determination of the stellar ages in different fields is provided through the comparison between the observations and synthetic stellar population models, which have been carefully tailored to account for the observational effects (i.e. distance dispersion, differential reddening, photometric completeness, photometric and systematic uncertainties). The simulations leading to the construction of synthetic populations have been carried out by using two different methods: i) a model that uses a spectroscopically derived metallicity distribution functions as prior, leaving the age as the only free parameter; ii) a genetic algorithm that finds the best solution within all possible combinations of age and metallicity (i.e. uniform prior in age and metallicity using IAC-POP/Minniac suite). We ultimately find that the bulge itself appears to be on average old (>9.5 Gyr) throughout its extension (|l| < 10° and -10° < b < +5°), with a mild gradient of about 0.16 Gyr/deg towards the Galactic center.

Abstract

In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in which the stars that contributed to the build-up of different regions of the Milky Way stellar halo formed. In this talk I will first give an overview of some of the main properties of the Milky Way stellar halo based on literature studies. I will then present results concerning the chemical properties of the outer regions of the Milky Way stellar halo, based on the elemental abundances of halo stars with large present-day Galactocentric distances, >15 kpc. The data-set we acquired consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28 red giant branch stars covering a wide metallicity range, -3.1 ≲ [Fe/H] ≲-0.6. We show that the ratio of α-elements over Fe as a function of [Fe/H] for our sample of outer halo stars is not dissimilar from the pattern shown by MW halo stars from solar neighborhood samples. On the other hand, significant differences appear at [Fe/H] ≳-1.5 when considering chemical abundance ratios such as [Ba/Fe], [Na/Fe], [Ni/Fe], [Eu/Fe], [Ba/Y]. Qualitatively, this type of chemical abundance trends are observed in massive dwarf galaxies, such as Sagittarius and the Large Magellanic Cloud. This appears to suggest a larger contribution in the outer halo of stars formed in an environment with high initial star formation rate and already polluted by asymptotic giant branch stars with respect to inner halo samples.

Abstract

The detection of chemical inhomogeities in the Galactic disk requires an oustanding precision in the abundance measurements and a thorough estimation of the uncertainties. So far, studies in alpha-elements in disk stars either do not reach the required precision, or comprise too small samples in the solar neighborhood. Thanks to the Apache Point Galactic Evolution Experiment (APOGEE), we have for the first time a large spectroscopic sample of about 100.000 disk stars, with data homogeneously obtained, reduced, and analyzed. Taking advantage of such database, we examine the distribution of oxygen-to-iron abundance ratio in stars across the Galactic disk. These data reveal that the square root of the star-to-star cosmic variance in the [O/Fe] at a given metallicity is about 0.03 to 0.04 dex in both the thin- and thick-disk populations. Measuring the spread in [O/Fe] and other abundance ratios can provide strong constraints for models of Galactic chemical evolution. In this talk we will describe how we arrived at this result, the calculation of uncertainties, and implications regarding the chemical evolution of the Galaxy.

Abstract

In this talk I present an overview of the structure, activity and goalsof the Gaia-ESO survey, a large public spectroscopic survey aimed at investigatingthe origin and formation history of our Galaxy by collecting high quality spectroscopyof representative samples (about 105 Milky Way stars) of all Galactic stellar populations,in the field and in clusters. Briefly, I discuss the most relevant results obtained so far.In particular, I present our study on the internal kinematics of Galactic globular clusters based on the radial estimates obtained from the survey complemented with ESO archive data.

Abstract

Gaia - the ESA cornerstone astrometric mission - was launched in December 2013, with the goal of censing the Milky Way population in a 6D space (positions and velocity) of 10^9 point-like obects, with errors 100-1000 times smaller than Hipparcos, with three color magnitudes and spectra as well. The scientific impact of its data will be large in many fields of astrophysics, from Galactic science, to Solar system objects, to stellar astrophysics, to galaxies and Quasars; from the distance ladder revision to fundamental physics. I will describe the mission concept, the scientific goals, and the present status of the mission, with special attention to the flux calibration of Gaia data.

Abstract

The structure, kinematics and stellar population of the Galactic bulge is very complex. Only three years ago the bulge was discovered to be X-shaped, a structure believed to originate from the dynamical instabilities of a disk, through the formation and posterior heating of a bar. The study of its kinematics reveals a cylindrical rotation, typical of a bar, suggesting the absence of a spheroidal component. Nevertheless, the bulge stellar population is old, has a radial metallicity gradient, and element ratio indicative of a short formation timescale. All these elements conflict with a simplistic view of the bulge as a heated bar, formed via "secular" evolution of a disk. I will review our knowledge of the bulge properties as traced by the 3D structure, kinematics, and chemical composition of its red clump stars.